1. Field of the Invention
The invention relates to a membrane drying apparatus for drying moist air which includes a housing that has an intake connection for incoming air to be dried and a discharge connection for discharging dried air.
2. The Prior Art
A membrane drying apparatus for moist air which includes a housing containing a tubular fabric strand bundle that is made up of tubular fabric strands composed of selective absorption material for extracting water vapor from compressed air to be dried by channeling the air through the fabric strands of the fabric strand bundle, and which also includes a scavenging air duct branching off from a point downstream of the fabric stand bundle and containing a throttle to allow expansion of the scavenging air, is well known. In such apparatus the scavenging air duct leads back to the fabric strand bundle inside the housing where expanded scavenging air passes along the outside of the tubular fabric strands in an opposite direction to that of the air stream inside the fabric strands, the scavenging air absorbing the diffused water vapor which has passed through the fabric strands and subsequently flowing to the outside air near the intake area of the fabric strand bundle. A self-acting shut-off valve or spring-loaded back pressure valve is installed near the discharge area which is controlled by the output air stream and is used as a flow sensor that also controls the scavenging air duct. The output air can be used as a working medium in many types of mechanical systems.
Such known membrane dryers provide a significantly decreased dew point through the expansion of the channeled scavenging air, whereby the scavenging air absorbs diffused moisture from the compressed air as the scavenging air flows in a reverse direction around the fabric strands. Through this method, the pressure dew point may be typically decreased by approximately 30.degree. to 50.degree. centigrade depending on pressure and scavenging air amount. One disadvantage with this type of apparatus, however, is the continuous need for scavenging air, which may be 15 to over 30% of the total air supply for the membrane dryer. Regardless of whether the output air is needed continuously or only partially, the need for scavenging air remains the same since it is gauged to the maximum airflow as long as the dryer is not turned off.
It is known to decrease the scavenging air supply by automatically controlling the output airflow and accordingly determining the individual scavenging air supply. However, an electronic device is necessary to accomplish this function, which is programmed by using the results of extensive measuring data and knowledge about typical performance of the membranes and module, which is the fabric strand bundle. Through pulsating of a valve that is operated by the electronic control device, only as much scavenging air is supplied as needed for drying of the diffused output air. A cost reduction can be thus obtained for the compressed air drying process, especially during periods of high performance demands or during highly changing demands for airflow. To accomplish this function, there is a pre-programmed electronic peripheral device required that is individually controlled by an outside power source, which in turn considerably increases costs for the entire apparatus. It also makes the apparatus more complicated and decreases operational safety.
A membrane dryer apparatus is disclosed in JP 09 957 043 A wherein a shut-off device which controls the scavenging air duct is actuated by the incoming air stream, thereby opening the scavenging air duct proportionally to the output airflow. By using this type of membrane dryer apparatus there is no need for outside controls and an outside power source. The scavenging air stream is adjusted independently according to the particular air requirement and is cut off when there is a lack of output air discharge. However, the scavenging air stream is always directly coupled with the output air stream in this type of design. It is particularly difficult in dryers with a high volume of airflow to design the control unit, the effective air sensor, and the scavenging air valve in a manner to function satisfactorily.
The object of the present invention is to adjust air consumption to the scavenging air requirement of the first mentioned type of membrane dryer apparatus in a simple way by using two phases, even at high volume air requirements, without a need for external intervention, outside energy sources or a complicated peripheral device.
The object is achieved by installing a separate scavenging airflow valve in the scavenging air duct, for example a 2/2-way directional control valve, that is actuated by the shut-off valve via a control line which branches off from the output air duct. This type of design is also suited for larger size membrane dryers whereby the shut-off valve simply opens and closes a control line with a relatively low airflow requirement. Such a control device is especially advantageous in decentralized dryers for protecting the equipment that is very sensitive and/or is exposed to large fluctuations in temperatures. In many cases, the air requirement does not fluctuate by much in such applications. However, such devices are only in operation at certain periods and with a delay in-between. The dryer equipment of this invention is of especially great advantage in such situations.
The scavenging air valve can be advantageously designed as a normally closed membrane valve which is biased to close by pressure of air flowing from a point downstream of the shut-off valve and is biased to open by pressure of air flowing from a point upstream of the shut-off valve. The pressure differential at the shut-off valve is used in this case to regulate the membrane valve, whereby the pressure differential depends largely on the amount of airflow. By having a relatively large membrane area compared to the smaller control area of the scavenging air duct, the amplification factor is affected in such a manner that a very simple scavenging airflow control may be attained.
According to the invention, the control line has a continuously open vent nozzle that ensures that during closing of the control line, the control line and therefore also the control side of the scavenging air valve is decompressed, thus allowing the scavenging valve to close. The vent nozzle should preferably open into the scavenging air duct downstream of its throttle.
Another aspect of the invention is for a bypass duct for the scavenging air to be located at the discharge side of the fabric strand bundle and placed upstream of the shut-off valve. An adjustable throttle can be installed in the bypass duct, which branches off from the output air duct and leads to the scavenging air duct. This small dimensional circumvention of the scavenging air line causes the membrane dryer to always need a very low amount of scavenging air, which also ensures removal of moisture from the fabric strand bundle even when the membrane dryer is in an operational mode below the tolerance level of the shut-off valve. The dryer will thereby remain at continuous operational readiness and will ensure that, with any new response to needs for drying and new airflow, the required dried air can be obtained immediately. During longer delays in dryer operation, moisture may diffuse through the tubular fabric strands because of compressed air within the fabric strands. This moisture is removed also during non-operational periods of the membrane dryer without considerable consumption of scavenging air so that the membrane dryer, according to the invention, remains continuously in operational readiness through this simple arrangement.